Electrical systems



R. J. HANAK 3,082,405

ELECTRICAL SYSTEMS 3 Sheets-Sheet 1 .March 19, 1963 Filed oct. 15. 1958March 19, 1963 R. J. HANAK ELECTRICAL sysTEMs Filed oct. 15, 195s 5Sheets-Sheet 2 R- J. HANAK ELECTRICAL SYSTEMS March 19, 1963 Filed oct.15, 1955 /A/Tf/w/Arf @IMX 27 3 Sheets-Sheet 25.4

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Unite This invention relates to electrical systems and in particular tosystems for actuating selected ones of a plurality of receivers to theexclusion of all other receivers.

In recent years there has been an increased demand for socalled pagingsystems, eg., systems by which a central station can communicate withselected individuals carrying portable receivers. The 'central stationtransmits certain signals which are received by the receivers andutilized to alert selected ones of the carriers of the receivers. Pagingsystems may |be of two general types, i.e., ones in which all receiversrespond tothe transmitted signal, or ones in which only selectedreceivers respond to the transmitted paging signals. It is with thelatter type of system, the selective paging system, that this inventionis concerned.

Selective paging systems ymay be categorized by their operativeprinciples; a few of the many types of selective paging systems will nowbe mentioned in order to clarify the advantages o-f the presentinvention. One type consists of a transmitter which sends out electricalsignals modulated at predetermined -frequencies which are received Iby anumber of receivers. Each receiver has a mechanically resonant reedwhich is tuned to a different modulation frequency. When theltransmitted modulation signal has a frequency to which a particularreed is resonant, the receiver containing said reed emits an audiblesignal which informs the carrier that he is to call the central stationto obtain the desired intelligence, or tells him to stand by for furtherinformation subsequently to be transmitted. These receivers, containingmechanical frequency selection apparatus, are subject to thedisadvantages common to mechanical systems. Thus, for example, physicalshocks and vibrations can result in false calls, temperature variationscan cause malfunction because of temperature sensitive components,mechanical wear causes the reeds to detune easily since there is loss ofmass in the resonant device due to such wear, standardization of the(frequency of a number of theresonant reeds having the same nominalnatural frequency is diicult to achieve, and the range of resonantfrequencies practically available is severely limited.

Another type of system employs an inductive loop usually located on theperiphery of Ithe callin-g region which, when energized, induces asignal in the receiver. The induced signal may be applied in thereceiver to a frequency discriminating circuit such as a resonant reedor a piezo-electric device. However, the disadvantages of resonant reedshave already been mentioned and piezoelectric crystals at the usualaudio frequencies are relatively large, are fragile, and some, such aschelate crystals, `cannot withstand high environmental temperatures. Theabove mentioned prior art systems involve a number of receive-rs eachadapted to be actuated by a different selected signal frequency. Suchsystems are practically limited in the number of receivers that may beselectively alerted.

The prior art also contains systems in which particular receivers areactuated in response to a group of transmitted pulses. However, if it isdesired to use pulse systems for general commercial use, the limitationsimposed by the Federal Communications Commission as to permissible sideband radiation are so rigorous that in practice they yconstituteeffective economic and technical barriers against their widespread use.

3,082,405 Eatented Mar.. 19, 1963 It is therefore a principal object ofthe present invention to provide selective paging systems which possessinherent operational and frequency stability.

Another object of the present invention is .to provide selective pagingsystems which permit the use of a much greater number of receivers thanis ordinarily possible with mechanically resonant-type devices.

Still another object of the invention is to provide selective pagingsystems in which frequency sensitivity of the frequency-selective partsof the receivers may be easily standardized.

Another object is to provide paging systems which are A very compact.

Another aim of the invention is to providea frequency discriminatingsystem( of much greater selectivity than has hitherto been available.

Still another object of the invention is to provide a frequencydiscriminating system whose bandwidth and/ or central frequency iseasily adjustable.

` In accordance with my invention I provide a transmitter /whichtransmits a group of signals having different frequencies, each of whichcorresponds to a certain character or digit of a predetermined selectedgroup of characters or digits that constitute a call number. Eachtransmitted signal will actuate those of the circuits in the receiverswhich are set therefor. When all the signals representative of thedigits of the call number lhave been transmitted, those of the receiverswhich have circuits responsive to all of the tones transmitted in theproper sequence will produce an audible or visible ale signal, forexample, which informs the carriers of those receivers that there is amessage to be received. All receivers have provision ttor enabling thealert signal to lbe shut olf when the `carrier of the receiver isalerted. The called carrier then communicates with a designatedswitchboard which discloses the message to him. In one form of theinvention I provide for the transmission of a so-called cancel signalafter the transmission of the tone representing the last digit of thenumber. This signal prevents false calls by disabling all circuits inall receivers which have circuits which have responded to some, but notall, of the tones of the just-called number.

The invention will now be explained in more detail. It should iirst beremarked that each tone transmitted by the Icentral station actuallyconsists of an RF carrier modulated by a selected audio frequency. Eachreceiver has provision for 1) demodulating the audio from the RFcarrier, (2) using each cycle'of the demodulated audio to vgenerate apulse, and (3) integrating the resultant chain of pulses, whoserepetition rate corresponds to the modulating audio frequency, to obtaina D.C. signal whose amplitude is uniquely a function of the audiofrequency. The integrated D C. signal is simultaneously supplied to `allof a number of switching or relay circuit-s that are found in eachreceiver. There is one switching circuit in each receiver for each digitof the group of numbers which identify a particular receiver so that ifthe call numbers have three digits, for example, each receiver will havethree switching circuits.

Each of the switching circuits is so designed that it can be actuatedonly when the integrated DC. volt-age applied to it is within a certainrange of amplitude values. Thus, since each transmitted tone will causethe production of a distinctively different integrated D.C. voltage itis seen that each switching circuit can respond only when a certain oneof the possible tones is transmitted.

The switching circuits in each receiver are connected serially and areso designed that when any one of them (except the last of the series) isactuated by a transmitted audio tone, it produces an output enablingsignal which must be applied to the next switching circuit in the seriesto enable the latter circuit to be actuated by a transmitted audio tonefor which said next circuit is set. The last circuit in the series,however, produces an output alert signal when the previous circuitapplies an enabling signal to it simultaneously with the applicationthereto of a transmitted tone for which said last circuit is set. Thisalert signal may consist of an audible signal like a ring, buzz, etc.,or a visual signal such as a flashing light, or may be used to turn onapparatus for receiving the ensuing message.

After the last of the tones has been transmitted, the cancel signal,which has a frequency which will cause each receiver to produce anintegrated D.C. voltage of such a value as to turn ol the first of theswitching circuits, is transmitted. When the first circuit is turnedoff, it no longer produces an enabling signal with the consequence thatthe following switching circuit, and the ones after that (except thelast) are similarly disabled. The last one is so constructed that onceit has begun to produce an alerting signal, it can only be deactivatedby a manual operation by the carrier of the receiver. Since all othercircuits of all other receivers have been cleared or reset by the cancelsignal they are again free to respond, or not respond, as the case maybe, to subsequently transmitted groups of transmitted tonesrepresentative of other call numbers. This cancel signal serves toprevent false calls as will be explained in some detail hereinafter.

The invention may be understood from a perusal of the drawings in which:

FIGURE l is a block diagram of an overall system constructed inaccordance with my invention;

FIGURE 2 is a schematic diagram illustrating the forms which certain ofthe components illustrated in FIG. 1 may take;

FIGURE 3 is a group of three graphs illustrating the operation of theapparatus shown in FIG. 2; and

FIGURES 4 and 5 are schematic circuit diagrams of still other componentsof the system shown in FIG. l.

Overall Operation of the System Referring to FIG. l a transmitter 11 islocated at a point near the calling area in which paging is to beeffective. The transmitter 11 may comprise conventional apparatus fortransmitting (l) an RF carrier which is modulated at different audiofrequencies to correspond to different digits and (2) the so-calledcancel signal. Let us assume, for purposes of illustration, that it isdesired to alert or summon the individual (or individuals) carrying areceiver which is set to respond to the number 756. The operator at thetransmitter 11 accordingly transmits, in sequence, a carrier Imodulatedat say, 7 kc., 5 kc., and 6 kc. In the right-hand part of FIG. l areshown the constituent components of the receivers used in conjunctionwith the transmitter 11.

The transmitted modulated carriers are applied, in sequence, to signalreceiving and demodulating circuits 15 which produce audio signals at 7kc., 5 kc. and 6 kc. respectively. Circuits 15 may comprise conventionalRF and audio demodulation circuits, or may consist of superregenerativedetection circuits, or other well-known equivalents. The detected audiosignals are applied in sequence to a conventional audio amplier 17whence they are applied to a clipper and dilerentiator 19 (which may beof conventional design) where substantially only the positive halfcycles of the audio signal are first extracted and then shaped by thediferentiator into positive and negative pips, the latter being used totrigger a oneshot multivibrator 21. The latter, which may be ofconventional construction, produces a positive-going rectangular pulsecorresponding to each clipped half-cycle as shown in the output of themultivibrator 21. It will be apparent that the higher the frequency ofthe demodulated audio is, the greater will be the number of rectangularpulses in the output of the .multivibrator 2. These pulses are thenapplied to an integrator 23, which may be of conventional design, whichis constructed to `produce an output D.C. voltage whose amplitude is afunction of l the frequency of the audio modulation of the signal thenbeing received. Hence, for each transmitted digit-representative signal,the output of the integrator at point K will have a unique amplitude.The output signal of integrator 23 is compared in voltage with referencevoltages supplied by the voltage reference circuit 31 as will beexplained in more detail in connection with FIGS. 2, 4 and 5. Circuit 3lis connected to each of the relays at the several terminals shown (C, H,L and their primed counterparts) for reasons which will be consideredbelow.

Operation of "Relays One of the most important features of the inventionresides in the switching circuits or relays 25, 27 and 29 all of whichreceive simultaneously at point K the output signals from integrator 23.Each of these relays has a number of characteristics in common with theother relays and a number of distinguishing features. Each relay may beconsidered to be voltage-sensitive, that is, it is so arranged that itwill turn on and produce an output signal only when the integratorsignal at K has a predetermined amplitude. Therefore each relay is, inessence, responsive only to the reception of a different one of thepossible transmitted audio-modulated-RF signals and hence may beconsidered as `being digit-responsive.

There are as many relays in each receiver as there are digits in thetransmitted call-numbers. For example, if only numbers in the hundredsare to be called there will be three relays such as the relays 25, 27,and 29 in each receiver. Relay 25 corresponds, and is responsive to, therst digit, relay 27 corresponds, andresponds to, the second digit, andrelay 29 corresponds to the third digit of a selected call-number. Inthe illustrative case the relay 2S would be set so as to be activatedwhen the 7 k.c. audio signal is transmitted, the relay 27 would be settorespond to the 5 k.c. audio signal, and the relay 29 would be set torespond to the 6 k.c. audio signal.

When the first digit-representative signal (7 kc.) is received theintegrator 23 will produce an output signal of say, x amplitude units(which is within the predetermined range) that is applied to inputterminal A of the rst relay 25. The latter thereupon is turned on andproduces an output enabling signal at output terminal O which, whenapplied to the B input of relay 27, conditions the latter to operatewhenever a 5 k.c. signal is transmitted and received.

When the second (5 lac.) signal is transmitted it causes the integrator23 to produce an output signal exceeding an amplitude value of say zunits, which is applied to the inputs A, A', and A of the relays.However, since only relay 27 is constructed so that it can be turned onby a signal having more than z amplitude and since it has received anenabling signal from the relay 25, it alone will be actuated by the (z)signal at input terminal A. As the two requisite conditions do co-exist,the relay 27 is turned on and, in turn, will provide an output orenabling signal to the B input of the last or 11th relay 29.

When the transmitter '11 sends out the 6 k.c. modulated carrier signalthe integrator 23 will produce an output signal exceeding a value of yamplitude units which is within the predetermined amplitude range thatwill cause relay 29 to be turned on when applied to the A input of thatrelay, since an enabling signal is already present at the B inputthereof. When this occurs the relay 29 will produce an output signalwhich actuates an altering device such as the audio oscillator 33, whichmay be of conventional design, whose output is connected to aloudspeaker35 Iwhich thereupon emits an audible alert tone. When the carrier ofthis particular receiver hears the alert signal he turns o his receiverand, for example, communicates with a pre-arranged station to ascertainwhat message there is for him. Since the relays of all other receiversare set for different call numbers, however, their respective alertingdevices will not be actuated and hence their carriers will not be awareof the fact that anyone is being called.

Since other receivers may also possess relays which are set to respondto some of the digits contained in the just-called number 756, e.g.,receivers set to respond to the number 563, and since the rst digit ofthe next number to be called may be a 3, it is necessary to clear allrelay circuits in all'receivers; otherwise the receiver set for callnumber 563 may respond erroneously to the next number called.Accordingly, a cancel signa is sent immediately after the transmissionof the signal corresponding to the last digit of the number just called.This cancel signal resembles the transmitted digit-representative signalexcept for the fact that its frequency is higher than any frequency usedto represent a digit. As was stated above, this cancel signal will turnolf all receivers except the one or ones responsive to the justcallednumber by turning olf the first switching circuit or relay in suchreceivers.

There is therefore provided in the first relay 25 of the just-calledreceiver as well as in the first relays of all other receivers acancelling circuit which, when the D.C. voltage at point K correspondsto the cancel signal, turns olf the relay 2S so that no output signal orenabling signal can be applied to the relay 27 thereby cutting off thelatter and its production of an enabling signal. However, since relay 29is so constructed that once it has been turned on it can be disabledonly by a manual operation, it is not atfected by reception of thecancel signal.

Operation of the First Relay FIGURE 2 shows a preferred form that thefirst relay 2S and the associated integrator 23 and voltage referencecircuit 31 may take. It is helpful to consider the first relay asconsisting of two main parts: (l) a part which produces aself-sustaining regenerative output signal and (2) a part whichdetermines if and when the relay will produce an output signal.

Part l of the relay 2S comprises the regenerative output-signalproducing part and includes TR1, TR2, R1, R0 and the battery 30". Inorder for the relay to produce an output signal the negative terminal ofbattery 30 must be connected, temporarily to the base of transistor TR1.

In actuality this is accomplished by turning on TRS aswill be explainedbelow in connection with the explanation of part 2. When this happensTRS will conduct and there will be a first current flowing from battery30 through the emitter and collector of TR1 causing the base of TR2 togo more positive. This will cause TR2 to conduct so that a secondcurrent will ow from the battery 30 through R1, through TR2, through theoutput resistor R0 and back to the battery. It is seen that the 'secondcurrent through R1 will tend to keep the base of TR1 negative therebycausing it to conduct so that TR2 will likewise conduct whereupon thedrop across R1 will tend to keep TR1 conductive. This condition isillustrated by the dashed-line curve 60 of part C of FIG. 3 which showsthe current or voltage through R1 (which is also representative of thecurrent through R0) attaining and then exceeding the critical valuebeyond which the current is self-sustaining. If the voltage across R1fails vto reach the critcal level within a specied time and TRS issubsequently cut off (part B, FIG. 3) the signal across R0 will not beself-sustaining. However, once the voltage across R1 attains thecritical level, the circuit of part l of relay 25 will produce a signalacross R0 which will continue even if the base of TR1 is subsequentlydisconnected from the negative terminal of the battery 30 if TRS is cutolf. R1 is preferably a negative-coefcient resistor, e.g., a thermistor,in order to stabilize the regenerative circuit for variations in ambienttemperature, although other known forms and methods of temperaturecompensation may alternatively be employed. For example, anotherthermistor may be placed between the base and emitter of TR2.

Part 2 of the relay comprises the circuits connected with transistorsTRS, TR4, and TRS. In order. to produce an output signal across R0, asstated above, a negative voltage must be applied from the battery 30 tobase of TR1. Therefore, if there is applied between K and Q a positiveD.C. Voltage which, when algebraically combined with x volts at terminalL, produces a negative voltage at the base of TRS, the latter willcommence conduction and the negative side of the battery 30 will beconnected to the base of TR1. It will be seen that the voltage between Kand Q must exceed x volts before there will be current ilow through R1which may, after the lapse of a predetermined interval of time, reachthe critical level (FIG. 3, part C) at which the signal across R0 isself-sustaining. However, until the current does reach the criticallevel, the voltage across R0 will be neither self-sustaining or largeenough to enable the subsequent relay 27 to be turned on should there besubsequently applied between K and L' an appropriate signal.

Once the critical level is reached, withdrawal or dirninution of thevoltage between K and Q cannot prevent the circuit of part l fromcontinuing to operate as explained previously. In such case, the voltageacross R0 will be suicient, when applied to the subsequent relay 27, toenable the latter to be turned on should the transmitter send out asignal modulated at 5 kc., the frequency for which relay 27 is set.

I have also constructed relay 2S so that if the voltage between K and Q,after having exceeded x volts, subsequently exceeds x' volts (FIG. 2)TRS will be rendered inoperative by virtue of the action of theanti-trigger circuit T'R4. It will be noted that the base of TR4 isconnected via resistor 26 to the terminal H which in turn is connectedto the terminal of the battery 31 at which x is obtained, the lattervoltage being of the same 'polarity but of greater magnitude than thevoltage x (with reference to Q). Thus if the voltage between K and Qexceeds x volts the transistor TR4 will conduct thereby effectivelyshorting the base of TRS to K. This will, of course, cut oit TRS andeffectively disconnect the base of TR1 from the negative terminal ofbattery Sti. The disconnection of the battery 30 from the base of TR1,however, may or 4may not prevent the prowithin the x-x range for whichit is set, but also would,

respond to the build-up to any greater voltage ofthe same polarity whenthe latter voltage temporarily had values in the aforesaid range. Thiswould happen inasmuch as the current through R1 would immediately excedthe `critical level. The converse is also true, i.e., when a voltagebetween K and Q in excess of x' was diminishing it might, whentemporarily in the x-x' range, cause relay 2S to produce an enablingsignal.

The circuit which prevents-the production of a selfsustaining enablingsignal by relay 2S unless the voltage between K and Q stays within thex-x range for at .least the minimum interval comprises thecapacitor vC1and the resistances of the various transistors in part 1 of the circuit.This time constant circuit prevents the voltage across R1 from attainingimmediately the critical value (part C, FIG. 3). The time constantcircuit does not affect the build up of the voltage applied between Aand Q even though the capacitor is connected to terminal A. Rather thetime constant circuit operates only in conjunction with the circuit ofpart l, i.e., TR1, TR2, R1, `R0 and battery 30, to cause the buildup ofcur-rent 7 through R1 to lag behind the voltage between A and the otherinput terminals.

The choice of the position of the time constant capacitor C1 and itscounterparts in the other relays is influenced by the fact that it isdesirable to employ the least value of capacitance which will enable thecircuit 2S to function properly. Accordingly, in the position shown inFIG. 2, the capacitor C1 may have a value of say, one microfarad which,in conjunction with the parameters of the transistors, gives a timeconstant between .O and 2 seconds. Alternatively, this capacitor can bedisposed between the emitter and the base of TR2 but this arrangementwill necessitate a greater value of capacitance than was required in thefirst arrangement.

Thus when the applied voltage at K and Q at the time to (part A, FIG. 3)begins to exceed x volts TRS will start to conduct (part B, FIG. 3)thereby causing some curernt to flow through R1 (part C).- It will beseen by reference to the solid line curve 61 that the current through R1at a subsequent predetermined time t1 is insuicient to cause thecritical voltage level across R1 to be attained. Also, at t1 the appliedvoltage (part A) starts to exceed x volts thereby causing theanti-trigger transistor TR4 to commence operation (part B) therebystarting to turn of TRS. The anti-trigger transistor TR4 is thus turnedon when the voltage between A and Q exceeds .x volts. When TR4 isconducting the voltage between K and X is diminished to a point whereTRS is effectively cut olf (part B, FIG. 3). This prevents the negativevoltage from battery 30 from being applied via TRS to the base of TR1and as a result the regenerative circuit (TR-l, TR2, etc.) will beinoperative and no self-sustaining output enabling signa across R0 willappear between terminals O and A.

The elements of relay 25 that have been described thus far are common toall the relays 2S, 27 and 29. There is, however, another part of thefirst relay circuit which is unique, i.e., the so-called cancel circuit.This circuit is the one which responds to the transmitted cancel signalby turning oi the first relay 2S which t thereupon produces no outputenabling signal and causes the next relay to turn off, and so on downthe line (with the exception of the last relay which can only bemanually turned oi) as has been previously explained. It responds to asignal higher in frequency than any other signal transmitted.

This circuit comprises the transistor'TRS which is set` to respond to avoltage applied between A and Q of say u volts which cor-responds to atransmitted 13KC cancel signal. Upon receiving this signal, TRS conductsand shorts out R1 which it shunts, reducing the negative voltage to TR1below the value required to maintain selfsustaining regeneration andcausing the regeneration to cease thereby preventing the production ofan output enabling signal across R0. It should be remembered that thedifference between the operation of the antitrigger transistor TR4 andthe cancel circuit transistor TR3 is that the latter can turn ott therelay 2-5 even after the regenerative signal-producing transistors TR1and TR2 are made conductive, whereas the anti-trigger transistor T R4cannot.

Description of Intermediate Relay The intermediate relay 27 shown inblock form in FIG. l is shown in schematic form in FIG. 4. As explainedpreviously, it contains many of the features of the tirst relay 25 andincludes trigger or low-limit, and anti-trigger or high-limit circuitsfor determining when it is to be turned on. Italso contains a part whichproduces a regenerative signal at the output.

The part of the circuit which regenerates comprises transistors TRla,TRZa, Rla and R0 which are the counterparts in the similarly numberedcomponents shown in FIG. 2. The trigger circuit comprises TRSa which isconnected via a base resistor to terminal L and serves the same functionas did TRS in FIG. 2. Thus when a certain trigger voltage in excess of zvolts is present between A and terminal Q, TRSa conducts so that acurrent flows from battery 30' and through resistor Rla causing thevoltage across Rla to increase until the base of TRla goes sufiicientlynegative that it conducts 4to the point where regeneration commencesresulting in the production of a self-sustaining output signal acrossthe output resistor R0 at the terminal O.

Transistor TR4a which is coupled to the terminal H via a base resistorserves the same function as its counterpart in FIG. 2, i.e., if thevoltage applied between K and terminal H exceeds the range of voltagesfor which the relay 27 is set to respond, it will prevent the inputsignal from causing the transistor TRS to conduct thereby preventingregeneration in the circuit and preventing the production of aself-sustaining output signal.

Transistor TRSa, which constitutes the enabling circuit, is connectedvia a base transistor to the output terminal O of the previous relay 2S.As was mentioned earlier in connection with the general description ofthe system of FIG. l, no intermediate or final relay can be operativeunless and until the previous relay is producing a self-sustainingregenerative output signal. So long as the rst relay 2S is producing anoutput signal which is applied to the base of TRSa, the terminal O willremain relatively positive, with respect to K, the transistor TRSrz willbe non-conductive, and an output signal can appear at O. Should theprevious relay not be actuated, however, there will appear between K andterminal O, the voltage which appears across the battery 3() in thepreceding stage. This voltage causes the transistor TR3a to conduct andhence the batery 39' has its positive side connected directly to Kthereby shorting resistor Rla and preventing regeneration in thecircuit. Attention is drawn to the fact that, unlike its counterpart,the cancel transistor TRS of FIG. 2 which is connected to a referencebattery, the base of the enabling transistor TR3a is connected to theoutput O of the previous relay.

Actually, the output voltage wave of the integrator 23 is compared, ineffect, before application to the relays 2S, 27 and 29 with referencevoltages in circuit 31. Since it is necessary for each of the relays torespond to different voltage levels, the base of TRS will be biased to acertain value (x volts), whereas the similar transistors TRSa and TRSbin the subsequent relays will be biased to respectively differentvoltages (z, y volts) so that each relay is, in essence, sensitive todifferent integrator output voltages at K. One way of accomplishing thisis by connecting batteries having selected voltage ratings in series (asshown in block 31, FIG. 2) and tapping off at different terminalsthereof. The output voltage of the integrator 23 is applied in series tothis chain of batteries and the differential voltage (i.e., thealgebraic sum of the battery voltage and the integrator output voltage)is the effective voltage across the trigger transistors TRS, TRSa andTRSb as the case may be.

The same sort of arrangement exists for determining the voltages atwhich the anti-trigger transistors TR4, TR4a, and TR4b will operate inrelays 2S, 27 and 29,

The nth Relay FIGURE 5 shows the constitution of the final or nth relay29. Those transistors shown therein which are identical to those shownin the preceding figures bear similar designations except for thesubscript b. TRlb and TRZb, together with the resistors Rlb and thelbattery 30" comprise theregenerative, output-signal producing circuit.TRSb is the trigger transistor and TR4!) prevents the relay 31 frombeing turned on if a higher than predetermined voltage is applied at K.

TR6, the enabling transistor, is connected differently and performssomewhat different functions than the transistors in the previousrelays. Its base is connected via a reaosaao'e sistor to the outputterminal O of the previous relay 27, and its emitter is `coupled to thebase of the trigger transistor TRSb (and to the `collector of TR4b).When the previous relay 27 is non-conductive the voltage between K andO' is suffi-cient to cause the transistor TR6 to conduct therebypreventing the trigger signal from being applied to the base of thetrigger transistor TRSc. When the previous relay 2-7 is on, however, thevoltage ,between K and lO decreases and the .transistor TRG isnon-conductive so that the proper trigger voltage can -be .applied ytothe base of TRSb to trigger the relay 29 to turn the latter onf Unlikethe enabling transistors of the intermediate relays, which can turn offthose relays once they have been turned on by shorting the resistor inseries with the base of TRla, TR6 cannot turn the relay off once it hasbeen on as it does not short out the resistor Rib when it is conductive.The only thing it can do is to by-pass the input signal at the base ofTRSb to K whenever the previous stage is inactive and thereby preventinitiation of operation of the relay 29. This arrangement isdeliberately made because it is desirable to require a manual operationto turn ofic the final relay (and hence the alert signal) after it hasonce been turned on.

For this purpose a manual reset switch 50, which shunts the resistorRlb, is provided. When it is closed by the carrier of the receiver thevoltage across Rllb is reduced so that the circuit 29 will no longerregenerate.

General Remarks While the invention has been described as a completesystem used for signalling purposes, its potential uses are many anddiverse. It has been demonstrated that the receiver has components whichrespond to different incoming modulating signal frequencies. It istherefore evident that it can be used as a frequency discriminatingand/'or detecting device. For frequencies in the lower ranges, i.e., upto about 450 kc. -it has definite advantages. ln the lower part of thatrange conventional filters of the LC type are toov large, require manycomponents, and require component changes in the event that `one desiresto change the frequency response thereof. By using the apparatusconstructed according to the present invention relatively few componentsare involved, the apparatus can be extremely small and compact, andthere is no -need to use different components if it is desired to changeto frequency to which it is to respond.

For example, all that is necessary to change the response frequency ofthe intermediate relay 27 from 5 kc. to another frequency is to changethe bias voltages on TRSa and TR4a by connecting to different points onthe voltage reference circuit 311. For changes of a substantial natureit may also be necessary to make an adjustment of component values inthe integrator 23. Incidentally, the circuit 311 is designed withseveral considerations in mind. The difference between the trigger andantitrigger voltages is governed by a number of factors. One of these,of course, is that this differential will depend upon the desiredselectivity and/or bandwidth to which the relay is to respond. Anotherconsideration is the fact that some of the components of the receivermay possibly be temperature-sensitive and cause minor variations infrequency response. Still another consideration is the frequencystability of the transmitter and the desired tolerances in the overallapparatus.

Also, if it is desired to change the passband of any one of the relaysit is only necessary to increase the difference between the voltagesapplied to the trigger and anti-trigger Itransistor bases. Conversely,if it is desired to narrow the passband, the difference between thevoltages applied to these bases can be made smaller. While FIG. 2 showsthat the circuit 31 is constructed so that there is .a differencebetween the voltage required to operate the anti-trigger circuit of onerelay and the trigger circuit of the next, it is entirely possible tohave them the 10 same thereby permitting the use of fewer batteries. Ofcourse, the entire battery circuit may be `supplanted by `appropriatevoltage divider networks .as are well known in the art.

It is also characteristic of these circuits that the passband curve hasextremely steep skirts which characteristic is especially valuable whenthese frequency discriminating circuits are to be used as switches as intelemetering applications where the device 4to -be controlled can beactivated or deactivated directly from the frequency responsive relay.Resort to conventional techniques to obtain these features wouldprobably necessitate employing expensive and complicated circuits.

The relay circuits have been shown as having certain transistors of thePNP type and others of the NPN type. It is apparent to those skilled inthe art that other circuit configurations are possible using NPNtransistors instead of PNP and vice versa. It is likewise apparent thatVcircuits other than the novel relay circuits shown and explainedpreviously can be used alternatively in their place, provided theyrespond to the voltages at the output of the integrator in the samemanner'. The novel circuits shown have proved eminently satisfactory forthe purpose, but circuits using tubes or other analogs of thetransistors undoubtedly could be devised to perform the same functionsin the system.

It should be remarked that with the system as shown, there should not bewithin any given receiver two successive relays which are `set torespond to the same voltage ranges, i.e., two relays which areresponsive to signals representing the same digit. By the same token',with the system as shown hereinbefore the transmitted call number shouldnot contain two successive identical digits. However, variations of thegeneral system as lshown are possible in which the callA number can havesuccessive characters which are the same.

This may be accomplished by resort to any one of several ways which arebased on variation in the duration of the time that eachdigit-representative signal is transmitted and/or variations in the timeconstants of the relays within any receiver. By way of illustration, onepossible arrangement could be to employ a receiver whose relays were setfor the digits 4, 4 and 5 respectively. The time constant of the firstrelay might be .l second and the time constant of the second relay mightbe .2 second for example. The transmitter could be designed to transmitvariable length bursts of sinusoidal Waves of the same frequency. Thus,in order to call 445 the operator would send out a first burst of a 4kc. modulated carrier having a duration of .l5 second (or more than .1second but less than .2 second) which would be of sufcient duration 'toturn on the first relay in the receiver which would thereupon produce anoutput signal for enabling the second relay. However, the first burstwould be too short to cause fthe :second relay to turn on despite theapplication thereto yof an enabling signal from the first relay.

To actuate the second relay of that receiver the operator next transmitsa burst of a 4 kc. modulated carrier having a duration of at least .2second. Since the first relay has already been turned on, it will ignorethe second burst, but the second relay will thereupon turn on and theoperator then needs only to transmit a 5 kc. modulated carrier to alertthe person carrying that receiver.

Another variation would be to employ identical time constants in the twosuccessive relays for responding to the 4-representative signal.Assuming this time constant was .l second, the operator would transmit afirst burst of a 4 kc. modulated carrier for between .l and .2 second.This would turn on the first relay which would then produce an enablingsignal that would be applied to the next relay. However, the latterrelay would not be turned on inasmuch as it has had the benelit of theenabling signal only after .l second has elapsed and then only for aperiodV shorter than the required .l second. However,

when the next burst arrives, the irst relay will ignore it as it isalready producing an enabling signal, but since it has a duration of atleast .1 second the second relay will turn on. Many other variations arepossible, as will be apparent to one skilled in the art.

The present invention is useful even in wired signalling systems inwhich case it is not necessary to employ a carrier for thedigit-representative signal. Where bandwidth and radiation are not aproblem, the transmitter could be made to send out different sets ofpulse trains which would correspond to the output of the multivibrator,and hence the preliminary parts of the receiver up to the integratorcould be dispensed with.

What I claim is:

1. A signalling system comprising: means for transmitting in apredetermined sequence a plurality of signal waves having differentfrequencies which respectively represent selected characters, means forreceiving said transmitted signal waves, said receiving means includingmeans responsive to said signal Waves for producing different voltagewaves respectively representative of the different frequencies of saidsignal waves, and means selectively responsive to said different voltagewaves for producing an output signal `only when said transmitted signalwaves have selected frequencies and occur in said sequence.

2. A signalling system comprising: means for transmitting in apredetermined sequence a plurality of signal Waves having differentfrequencies corresponding respectively to selected characters of a callnumber, means for receiving said transmitted signal waves whichcomprises means responsive to said signal waves for producing aplurality of sets of pulses, each set having a repetition rate whichcorresponds to one of said signal wave frequencies, means responsive tosaid plurality of sets of pulses for producing a plurality of differentvoltage Waves corresponding respectively to said sets of pulses, andmeans selectively responsive to said voltage waves for producing anoutput signal only when said transmitted signal waves have selectedfrequencies and occur in said predetermined sequence.

3. A signalling system comprising: means for transmitting in apredetermined sequence a plurality of sinusoidal waves having differentfrequencies corresponding respectively to different characters of apredetermined group of characters, and means for receiving saidtransmitted sinusoidal waves which includes means for clipping partcycles of said waves, means for producing sets of pulses ofsubstantially uniform amplitude, each of said pulses corresponding toone of said clipped part cycles, means for integrating said sets ofpulses to produce different values of voltage which are respectivefunctions of the repetition rates of said sets of pulses, a plurality ofswitching means, one for each character of said groups of characters,said plurality of switching means being constructed to receive saidvoltages and to produce an output signal only when said voltages havepredetermined values corresponding to selected ones of saidpredetermined groups of characters and occur in the same relativesequence as said transmitted waves.

4. A signalling system comprising: means for transmitting in apredetermined sequence a plurality of signal waves representing selectednumbers of a series of call numbers, each `of said signal wavescomprising a carrier wave modulated at a different audio frequency, anda receiver for said transmitted signal waves which includes means fordemodulating the audio component of said transmitted waves, means forclipping substantially half cycles from said demodulated audiocomponent, means for producing a plurality of sets of substantiallyrectangular pulses having substantially uniform amplitude, each pulse ofeach of said sets corresponding to one of said substantially halfcycles, means for integrating thewpulses of each set to producerespective output voltage waves which have amplitude values which arefunctions of the repetition rates of said pulses, and a plurality ofswitching circuits being constructed to produce an output signal onlywhen the applied voltage waves have amplitude values which correspond totransmitted signal Waves representing selected call numbers and occur insaid predetermined sequence.

5. A system for receiving a plurality of transmitted signals havingdifferent frequencies which respectively represent different charactersand occur in a predetermined sequence, said system comprising means forproducing a plurality of voltage waves in response to said transmittedsignals, each of said voltage waves representing one of said transmittedsignals, and means selectively responsive to said plurality of voltagewaves for producing an output signal only when said transmitted signalshave selected4 frequencies which occur in said sequence.

6. A system for receiving a plurality of different signal waves havingrespectively different frequencies corresponding to different selectedcharacters, said system compn'sing means responsive to said signal wavesfor producing corresponding different sets of pulses having respectiverepetition rates related to the respective frequencies of said signalwaves, means responsive to said different sets of pulses for producingrespective voltage waves corresponding thereto, and means selectivelyresponsive to said voltage waves for producing an output signal onlywhen said waves have respective predetermined ranges of amplitude andoccur in a predetermined sequence.

7. A system for receiving a plurality of different sinusoidal wavestransmitted in a predetermined sequence having respectively differentfrequencies corresponding to different characters of a series of callnumbers, said system comprising: means to which said sinusoidal wavesare applied for clipping substantially half cycles of said waves, meansresponsive to said clipped half cycles for producing sets of rectangularpulses of substantially uniform amplitude, the number of pulses of eachset corresponding to the number of said clipped half cycles, means forintegrating said sets of pulses thereby to produce different voltagewaves having amplitude values which are functions of the repetitionrates of the said sets of pulses, and means responsive to said differentvoltage waves for producing an output signal only when said voltageWaves have respective amplitudes which fall within predetermined rangesand occur ina sequence corresponding to said predetermined sequence.

8. The receiver according to claim 7 wherein said means for producingsaid output signal comprises a plurality of means each of which isresponsive to a different selected one of said voltage waves, therebeing one of said last-mentioned means for each character of selectedones of the series of call numbers, said plurality of voltageresponsivemeans being further constructed and arranged to receive each of saidvoltage waves and to produce an output signal substantially only whenselected ones of said voltage Waves have amplitudes within selectedpredetermined ranges.

9. A signalling system comprising means for transmitting a plurality ofsignal waves having selected frequencies respectively corresponding toselected characters of desired call numbers, said transmitting meansbeing constructed to transmit signal waves corresponding to twosuccessive identical characters of a selected call number forpredetermined different time intervals, and means for receiving saidtransmitted signal waves which comprises means for producing a set ofpulses in response to each of said signal waves, the repetition rateofeach set corresponding to one of said selected frequencies, saidreceiving means also comprising means for producing a voltage wave inresponse to each of said sets of pulses, the amplitude of said voltagewave being a function of the repetition rate of the set of pulses towhich it corresponds, and a plurality of circuits having substantiallythe same time constants for producing cooperatively an output signalsubstantially only when said voltage waves have selected amplitudevalues.

l0. A signalling system comprising means for transmitting a plurality`of signal Waves having selected frequencies respectively correspondingto selected characters of desired call numbers, said transmitting meansbeing constructed to transmit signal waves corresponding to twosuccessive identical characters of a selected call number forpredetermined different time intervals, and means for receiving saidtransmitted signal waves which comprises means for producing a set ofpulses in response to each of said signal waves, the repetition rate ofeach set corresponding to one of said selected frequencies, saidreceiving means also comprising means for producing a voltage wave inresponse to each of said sets of pulses, the amplitude of said voltageWave being a function of the repetition rate of the set yof pulses towhich it corresponds, and a plurality of circuits for producingcooperatively an output signal substantially only when said voltageWaves have selected amplitude values, those of said circuits which are14 directly connected to one another and set to respond to voltage waveshaving the same amplitude values being constructed to respond theretoonly when said same voltage Waves are applied thereto for predetermineddifferent time intervals.

References Cited in the le of this patent UNITED STATES PATENTS1,900,095 Brownstein Mar. 7, 1933 2,454,780 Deakin Nov. 30, 19482,547,023 Lense et al Apr. 3, 1951 2,559,622 Hildyard July 10, 19512,591,937 Herrick Apr. 8, 1952 2,600,405 Hoeppner .lune 17, 19522,617,872 Herrick Nov. 11, 1952 2,642,527 Kelley June 16, 1953 2,663,806Darlington Dec. 22, 1953 2,724,780 Harris Nov. 22, 1955

1. A SIGNALLING SYSTEM COMPRISING: MEANS FOR TRANSMITTING IN APREDETERMINED SEQUENCE A PLURALITY OF SIGNAL WAVES HAVING DIFFERENTFREQUENCIES WHICH RESPECTIVELY REPRESENT SELECTED CHARACTERS, MEANS FORRECEIVING SAID TRANSMITTED SIGNAL WAVES, SAID RECEIVING MEANS INCLUDINGMEANS RESPONSIVE TO SAID SIGNAL WAVES FOR PRODUCING DIFFERENT VOLTAGEWAVES RESPECTIVELY REPRESENTATIVE OF THE DIFFERENT FREQUENCIES OF SAIDSIGNAL WAVES, AND MEANS SELECTIVELY RESPONSIVE TO SAID DIFFERENT VOLTAGEWAVES FOR PRODUCING AN OUTPUT SIGNAL ONLY WHEN SAID TRANSMITTED SIGNALWAVES HAVE SELECTED FREQUENCIES AND OCCUR IN SAID SEQUENCE.